Introduction to Principles and Practice of Mass Spectrometry

1 History of Mass Spectrometry

According to the International Union of Pure and Applied Chemistry definition, 'Mass spectrometry is the branch of science dealing with all aspects of mass spectroscopes and the results obtained with these instruments.' It evolved from research in particle physics at the turn of the century, with Goldstein discovering positively charged 'rays' in 1886 and Wien (1898) studying their electric and magnetic properties. In the early 1900s J.J. Thomson built his 'parabola mass spectrograph' to measure the charge to mass ratio (z/m) for several ionic species. In the expression z/m, z is the charge number, i.e. the total charge on an ion divided by the elementary charge (e), and m is the nucleon number, i.e. the sum of the total number of protons and neutrons in an atom, molecule or ion. In modern mass spectrometry, the parameter measured is m/z, rather than z/m: the unit of m/z was recently designated the thomson (Th).

Aston continued the work at Cambridge and built instruments that helped him to establish the presence of isotopes. He was subsequently able to measure the atomic mass of most elements with sufficient accuracy to be able to calculate the 'packing fraction' of their atomic nuclei. The packing fraction is the difference between the accurate atomic mass of the isotope and the nearest whole number divided by the mass number, also known as the mass defect. Aston also obtained accurate measurements of the ratios of the stable isotopes of many of the known elements.

At the end of this exciting period of development, Aston was convinced that much of the potential of mass spectrometry had been exploited. It was not until the 1940s that the technique was put to work in elucidating organic structures in the petroleum industry. Ionisation was effected by electron 'impact' [now called electron ionisation (EI)] for those molecules that could withstand vaporisation into the heated and evacuated ion source without decomposition. This limited the practical mass range to less than 1000 daltons (Da) but yielded useful fragmentations for structure elucidation (see Chapter 2). By choosing to work with 70 electron volt (eV) electrons many ions were formed with internal energies far in excess of the ionisation energy (IE)]. These ions decompose rapidly to produce lower mass (fragment) ions and neutral radicals or molecules.

During the 1950s, commercial instruments were being built and new applications discovered. One of the earliest of these was the identification of low molecular weight volatile food flavour compounds. Ten years later, the powerful combination of electron ionisation mass spectrometry (EIMS) with gas chromatography (GC/MS) led to an explosion of applications where mass spectrometry was used in qualitative and quantitative, chemical and biochemical studies. GC/MS instruments produced enormous amounts of data, which were best handled by computers, and data acquisition and processing methods were devised. In 1966, Munson and Field described chemical ionisation (CI). This technique increased the yield of ions representative of the molecular weight of volatile molecules through interactions with reagent gas ions (e.g. CH+5 ions from methane) with little excess energy. Other 'soft' ionisation techniques such as field desorption (FD) and particle desorption methods based upon ion generation by Cf-252 fast fission products [plasma desorption, (PDMS)] were introduced during the 1970s for in volatile compounds. At the same time (and in response to these developments) the instrumental mass range was increased to cope with the larger sample molecule ions now entering the gas phase. This process accelerated in the 1980s with the introduction of Fast Atom Bombardment (FAB) ionisation. FAB was the first ionisation technique to enable biologists and biochemists routinely to obtain molecular weight information on complex, labile biomolecules, including polypeptides and small proteins.

Ionisation from the liquid state, followed by evaporation/desolvation of charged droplets, includes techniques such as ion spray, thermospray (TSP) and electrospray ionisation (ESI). These methods differed mainly in the manner in which ionisation was initiated. Multiply charged molecular ions could be formed under ESI, facilitating the measurement of high molecular masses, even on conventional instruments (i.e. those with a mass range up to 2000 or 4000 Th). More efficient pumping systems were required to cope with the increased gas volumes generated by vaporising liquids.

Separation techniques such as liquid chromatography and capillary electrophoresis coupled to mass spectrometry (LC/MS and CE/MS respectively) have extended the advantages first associated with the analysis of volatile compounds by GC/MS to compounds of low volatility and high molecular weight. Tandem mass spectrometry (MS/MS) collision-induced dissociation (CID), focal-plane array detectors, ion traps and hybrid instruments are providing a high sensitivity structure elucidation facility for involatile compounds similar to that provided by EIMS of volatiles. Recently, Laser Desorption (LD), and especially Matrix Assisted Laser Desorption Ionisation (MALDI), combined with Time-of-Flight (ToF) mass analysis has extended the practical mass range to over 300 000 Da, producing mainly singly-charged molecular ions. Fourier transform mass spectrometry (FTMS), also known as Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is only slowly entering the commercial area.

Only a brief introduction to the principles and practice of mass spectrometry is given herein. The reader is directed towards more general textbooks, e.g. Throck Watson or Rose and Johnstone, for a more detailed discussion of the principles and practice of mass spectrometry. Beynon and Brenton lucidly introduce the physical aspects of mass spectrometry and ionisation in the gas phase. Finally, the latest volume by McLafferty and Turecek is recommended for a very thorough introduction to the interpretation of organic mass spectra.

Mass Spectrometry – a definition

Mass spectrometry is the study of systems causing the formation of gaseous ions, with or without fragmentation, which are then characterised by their mass to charge ratios (m/z) and relative abundances.

Mass spectrometry is unlike most other forms of spectroscopy or spectrometry that are concerned with non-destructive interactions between molecules and electromagnetic radiation. This is because mass spectrometry is the...